1. Field of the Invention
The present invention relates to color couplers for photography and particularly to novel 2-equivalent color couplers and color photographic sensitive materials containing such color couplers.
2. Description of the Prior Art
It is well known that oxidized aromatic primary amine developing agents react with dye forming couplers to form color images by color development after exposure of silver halide photographic sensitive materials to light. In this case, a color reproduction process according to a conventional subtractive process is utilized to form cyan, magenta and yellow color images which are in a complementary relation with red, green and blue colors. For example, phenol derivatives or naphthol derivatives are used as couplers for forming a cyan color image and 5-pyrazolone derivatives, pyrazolobenzimidazole derivatives or imidazolone derivatives are used for forming a magenta color images.
In color photographic processes, the color forming couplers which are added to a developer or incorporated in photosensitive photographic emulsion layers or other color forming layers react with an oxidation product of the color developing agent formed by development to form nondiffusible color images. Almost all conventional cyan dye forming couplers and magenta color forming couplers are 4-equivalent couplers, that is, theoretically, 4-equivalent couplers stoichiometrically require 4 moles of exposed silver halide as an oxidizing agent for formation of 1 mol of the dye. Incorporation of a large amount of silver halide in the photosensitive layer is disadvantageous because the sharpness of the images formed deteriorates because of the increase of scattered light in the emulsion layer or because the rate of processing the photosensitive materials is reduced due to the increase in the thickness of the emulsion layers. Further, the formation of the dyes by these 4-equivalent couplers has the disadvantage that a strong oxidizing agent must be employed in the subsequent processing steps, because the complete formation of the dyes does not occur in a color development bath.
In order to overcome these disadvantages, use of 2-equivalent couplers, that is, couplers which require only 2 mols of exposed silver halide for forming 1 mol of the dye, has been proposed.
Two-equivalent couplers have a structure in which the coupling position, such as the p-position of a phenolic hydroxyl group or a hydrogen atom of an active methylene group in the 4-position of a 5-pyrazolone, is substituted with a releasable group. Examples of such releasable groups are sulfonamido groups as described in U.S. Pat. No. 3,737,316, imido groups as described in U.S. Pat. No. 3,749,735, sulfonyl groups as described in U.S. Pat. No. 3,622,328, aryloxy groups as described in U.S. Pat. No. 3,476,563, aryloxy groups as described in U.S. Pat. No. 3,419,391 and thiocyano groups as described in U.S. Pat. Nos. 3,214,377 and 3,253,924.
However, these known couplers are not always satisfactory, because they have the disadvantages that the coupling reactivity is insufficient, that the dispersibility is inferior resulting in difficulties on coating, that the synthesis of the couplers is very difficult, or that the storability of the color images formed is poor, etc.